1. Field of the Invention
The present invention relates to a hydraulic pressure valve with direct control.
2. Description of the Related Art
Hydraulic pressure valves known in the past have several significant disadvantages with respect to their construction and operation. On the one hand, directly controlled pressure valves, such as conical seat valves with pretensioned spring or proportional magnet, are only suitable for low hydraulic powers because the valve cones are without pressure compensation and, thus, very high pressure forces occur in the case of larger cone diameters. In addition, forces of flow act on the valve cone which forces enter the force balance of the cone as disturbance variable and which consequently lead to substantial deviations. On the other hand, pilot-controlled pressure valves whose pilot step usually is a directly controlled pressure valve make it possible to control substantially greater hydraulic powers, however, these valves also have severe disadvantages. The forces acting on the main step piston produce deviations in the balanced pressure behavior in addition to the deviations of the pilot, so that the stationary pressure/flow characteristics usually have an increasing tendency. Moreover, the dynamics of the pilot-controlled valves depend on the pilot pressure, i.e., the valves have substantially reduced dynamics when the available pilot pressures are small. Additional disadvantages of the pilot-controlled valves are the fact that the valves are of complicated construction and difficult to manufacture and they are sensitive to contamination.
In order to able to construct a directly controlled pressure valve, it is necessary to minimize the disturbance forces acting on the valve-piston, particularly the flow forces, because these forces enter directly as a disturbance variable the force balance determining the pressure to be controlled.
Various measures for effecting the compensation of the flow forces in hydraulic valves are known from the literature in this field, for example, from the dissertation entitled "Stromungskraftkompensation in direktgesteuerten, elektrohydraulischen Stetigventilen" Flow force compensation in directly controlled, electrohydraulic continuous valves!, RWTH Aachen 1992, by H.-J. Feigel. However, these proposed measures are not capable of achieving a compensation of the forces over the entire work range which would be sufficient for the case of application of the pressure valves. In the records of the Eleventh Fluid-technological Colloquium in Aachen 1994, C. Latour proposes on pages 35-50 a hydraulic two-way cartridge element which is essentially of the same type as the valve according to the present invention. This known element is essentially composed of an outer valve sleeve arranged in a port, a cylindrical, axially fixed inflow component and a cup-shaped, longitudinally movable tubular piston. This cartridge element is based on the concept of having the acceleration and deflection of the flow occurring in front of the control edge and the resulting change of the static pressure not act on the moving valve element but on the axially fixed valve element. As a result, in the state in which the flow force is not compensated, only very low flow forces act on the tubular piston. These remaining low forces are compensated by a deflection of the open jet, wherein the pressure medium jet is divided behind the control edge of the valve. A portion of this jet flows off directly radially through the outlet openings provided in the valve sleeve. The remaining portion is conducted into the deflection chamber by the webs formed between the outlet openings, wherein the pressure medium jet is approximately spirally deflected in the deflection chamber and the jet subsequently leaves the chamber through the outlet openings. The deflection of the pressure medium jet in the deflection chamber acts at least partially on the cup-shaped tubular piston in a direction which tends to open the low resistance, so that, when the deflection chamber is of suitable configuration, no significant flow forces occur over a large work area. Another significant advantage of this flow resistance with cup-shaped tubular piston is the fact that the tubular piston is completely force-balanced in the closed state even without a pressure compensating bore.